This invention is in the field of metal-insulator-semiconductor (MIS) sensors, in particular, MIS-based sensors with improved selectivity for hydrogen. Real-time hydrogen (H2) detection is important in various industrial settings. For example, H2 detection in the headspace of distribution transformers is an indicator of undesirable partial discharge.
The detection of gases dissolved in liquids is also an important problem for many applications. (Hedborg, E.; Winquist, F.; Lundstrom, I.; Andersson, L. I.; Mosbach, K., Some studies of molecularly-imprinted polymer membranes in combination with field-effect devices, Sens. Actuators A 1993, 37-8, 796). For example, faults may occur when oil-filled electrical equipment is subjected to thermal or electrical stresses, which break down the oil and generate “fault gases”.
Solid-state metal-insulator-semiconductor (MIS) sensors are excellent candidates for this purpose. These sensors are prepared by depositing a film of catalytic metal onto a thin layer of gate insulator material that has been grown on a semiconductor substrate. If the insulator is silicon dioxide, these sensors may also be termed metal-silicon dioxide-semiconductor (MOS) or thin-film metal-silicon dioxide-semiconductor (TMOS) sensors.
The basic H2 sensing mechanism for MIS devices has been described in pioneering works by Lundström and coworkers. To provide a brief description, gas-phase H2 dissociates on the surface of the catalytic metal to form H atoms. These H atoms then rapidly diffuse through the metal film to the metal-insulator interface, where they are preferentially trapped in stabilized adsorption sites. The layer of interfacial hydrogen created by this process exists in a dipole layer, creating an additional voltage drop across the MIS sensor that can be measured as either a voltage shift in the capacitance-voltage (C-V) curve of a capacitor, or in the current-voltage characteristic of a diode or transistor.
Previous work has proven that MIS sensors that are highly responsive to H2 can be reproducibly prepared. For example, MIS sensors are capable of detecting H2 concentrations in the parts per billion range. However, the development of MIS sensors for industrial applications is still challenging because of the cross-sensitivity to other gases, such as carbon monoxide (CO), ethylene (C2H4), acetylene (C2H2) and potentially oxygen (O2). Some of these gases, such as ethylene, are hydrogen containing gases which can dissociate to form hydrogen which can be directly detected by the sensor. Other gases are not directly detected by the sensor, but can influence the response of the sensor to hydrogen. For example, carbon monoxide can affect the response of the sensor to hydrogen and other gases through strong competitive adsorption on the catalytic metal surface. As another example, oxygen can affect the response of the sensor to hydrogen through water forming reactions. Water forming reactions between adsorbed H and O on the catalytic metal surface can consume H, leading to a substantially decreased sensor response.
Gas sensors including polymer coatings have been reported in the patent literature. U.S. Pat. No. 6,634,213 to O'Connor et al. reports a hydrogen permeable protective coating for a single-chip hydrogen sensor. Organic coatings are stated to be preferred; FLARE™ and HOSP™ spin-on coatings are specifically mentioned. U.S. Pat. No. 6,182,500 to Stokes et al. reports a protective layer disposed on the catalytic metal gate of a gas sensor. A thin film of hydrophobic polytetrafluoroethylene is mentioned as an exemplary material for the protective layer. The protective layer is stated to improve the sensor's sensitivity by allowing only certain gases to pass through and interact with the catalytic metal gate layer. Amounts of other gases, as well as foreign matter, that pass through the protective layer are stated to be reduced, and even prevented, from passing through. U.S. Pat. No. 6,895,805 to Hoagland reports a hydrogen gas indicator that comprises a gas diffusion barrier coupled to a catalyst material of a gas sensor; the gas sensor does not appear to be a metal-insulator-semiconductor sensor. The gas diffusion barrier is stated to allow selectively permeable diffusion of molecular hydrogen gas or atomic hydrogen gas.
There remains a need in the art for MIS sensors having improved hydrogen selectivity.